This program is designed to provide a multidisciplined approach to the study of flow in the microcirculation. Using a self-contained prototype microvascular bed identified in cat mesentery, the study undertakes to correlate flow dynamics in a representative sample of the bed with the exchange of a substance between the tissue and the bed measured simultaneously but independently. Primary measurements are made from photographic data obtained with a wide field reflecting objective designed for use on an intravital microscope built for the program. Detection of the temporal-spatial distribution of exchanging molecules will be accomplished optically both by absorption of visible light and by fluorescent emissions. Tracers will comprise small lipid insoluble molecules and gradations of molecular size up to serum albumin. The results from the simpler model will be extended to evaluate beds in other tissues.